Metallacycle polymerization

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

From the results discussed so far, it is evident that only CH2 groups have been observed in the very early stages of the ethylene polymerization reaction. Of course, this could be due to formation of metallacycles, but can be also a consequence of the high TOP which makes the observation of the first species troublesome. To better focalize the problem it is useful to present a concise review of the models proposed in the literature for ethylene coordination, initiation, and propagation reactions. 

Confirmation that the polymerizations proceed via metallacyclic intermediates was obtained by studying the ROMP of functionalized 7-oxanorbornadienes. These polymerize slower than their norbornene analogs, allowing NMR identification of the metallacyclobutane resonances and subsequent monitoring of ring opening to the first insertion product. In addition, the X-ray crystallographic structure of complex (212) has been reported.533... 

Examples of heavier alkali metal complexes include [ GH(SiMe3)(SiMe2OMe) M] (M = Na 59, K 60) as well as the polymeric etherate [CH(SiMe3)(SiMe2OMe)K(OEt2)]oo 61.69 All these examples demonstrate the potency of intramolecular coordination, since methoxide-metal interactions under formation of metallacycles are observed in all cases. 

As shown in Table 4.38, three major reaction pathways are available to hypova-lent metals in the presence of an alkene (A) and (C) dative and synergistic coordination (B) carbocation formation and (D) and (E) metallacyclic and migratory insertions. The latter types are of particular importance in metal-catalyzed alkene polymerizations and will be given primary attention in the discussion that follows. 

An example is provided by the structures of Group IV metallacycles, LL MCl2, where ligands L and L are cyclopentadienyl based and M is Ti or Zr. As a class, these compounds act as catalyst precursors in homogenous (Ziegler-Natta) polymerization of olefins, e.g. 

In contrast to the oligomers formed by intermolecularly connected AT -N dative bonds, a series of monomeric metallacycles was synthesized and their XRD structures were determined (Figure 17).111-119 Several five-membered structures are comprised of one Al-G covalent and one AT -N dative bond provided by the bidentate ligand conformation. Their potential ability as candidates for co-catalysts in MgC /TiCLpmediated polymerization was also demonstrated (Figure 17).83 For the propene polymerization, all co-catalysts reach activities in the range of Et3Al... 

The mechanism of this polymerization has been discussed by Cundy et al. (9). The first step is apparently the insertion of a low-valent metal into the strained C—Si bond to give a silametallacyclopentane. Metallacycles 4 and 5 have in fact been isolated from the reactions of Fe2(CO)9 and CpMn(CO)3 with 1, respectively (10, 11). Complex 4 when treated with phosphines gives polymer and LFe(CO>4. If the metallacycle resulting from insertion (6) is unstable, repeated insertions (oxidative additions) and reductive eliminations lead to polymer. Chain termination results from reductive elimination of =SiH [Eq. (13)]. 

Fig. 21, no absorption characterizing CH3 terminal groups is evident, even in the early stages of polymerization. Analogously, no spectroscopic evidence of carbene species could be found. Therefore, the chains were suggested to have a cyclic structure and to be very long even after short polymerization times (205,233), so that the metallacycle mechanism (I in Scheme 11) was considered the most probable. 

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